Stephens will play Elena Vesnina of Russia in Friday's other semifinal. Vesnina played better in windy conditions Thursday to beat wild-card Jarmila Gajdosova of Australia 6-3, 6-2.

"I had to be smart and in the wind you have to use it like a weapon," Vesnina said.

The 22-year-old Barthel said she's "improved a lot" as she begins her second season on the WTA Tour.

"I feel like I learned so much last year, playing all these tournaments and playing against the top players," Barthel said. "My serve has got better and I feel like my groundstrokes are better ... but maybe a little more confidence this year."